Charge pump circuits

ABSTRACT

Charge pump circuits having circuit components such as transistors which may be damaged by voltage transients greater than the normal operating voltage levels of the charge pump circuit, such as may be experienced during powering down. The circuit components to be protected are connected in parallel with a leakage element arranged to have a leakage current that is small enough during normal operation to allow the charge pump to operate effectively but which is large enough, during development of a voltage transient, to prevent excess voltage levels being achieved. The leakage element may have a significant leakage current at a voltage less than the breakdown voltage of the circuit component. Suitable leakage elements are poly diodes.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to charge pump circuits having leakageelements arranged to act as protective shunt elements across low-voltagesemiconductor devices, and in particular, but not exclusively, to suchcircuits having high impedance diodes with current leakagecharacteristics.

2. Description of the Related Art

Capacitive charge pumps and their use in generating higher voltages froma lower-voltage power supply are well known. Their use is increasing dueto the proliferation of battery-powered consumer gadgets such as MP3players and mobile phones. For example one use is to provide a voltagefor MEMS microphone transducers, where a 12V transducer bias voltage mayneed to be derived from a supply as low as 1.5V.

FIG. 1 a illustrates a general capacitive charge pump comprising fourpumped capacitors C1 to C4 interposed between switching stages SS1 toSS4. In operation, the voltages on the bottom plates of C1 to C4 arepulsed between an input voltage Vin and ground in alternate clockphases. The top plate of each capacitor is also connected to the topplate of an adjacent capacitor via a switching stage SS2-4. The firstcapacitor in the chain is also linked via switching stage SS1 to anothervoltage signal which is also conveniently equal to Vin. Each capacitortop plate cycles between a respective pair of voltages, with thesevoltages increasing along the chain, in this case delivering an outputvoltage of 6V to the last, unswitched, capacitor C4.

The switch elements may be simple diodes, as in the well known Dicksoncharge pump, but for higher efficiency and a more accurate outputvoltage they may be MOS switch transistors, driven, via level shiftcircuits LS1 to LS4 by another set of clock pulses to connect adjacentpairs of capacitors together in alternate clock phases.

FIG. 1 b illustrates the voltage levels at the nodes of the switchingstages with the solid and dashed lines indicating the voltage levelsachieved during different stages of the clock cycle. Thus the voltage atVin is constant. However the voltage at V2 alternates between Vin and2Vin. Similarly the voltage at V3 varies between 2Vin as a low voltageand 3Vin as a high voltage, but with the pattern of high and low voltagebeing of opposite phase to that at node V2. It can be seen from FIG. 1 bthat the voltage difference across each switch element when off neverexceeds twice the input voltage Vin. It is thus possible to constructsuch a charge pump using devices designed only to operate with up to2.Vin across them. This has advantages in that these devices are smallerthan higher voltage devices, so occupy less chip area and need lesscharge to turn on and off each clock cycle. In many applications, suchas the above-mentioned MEMS microphone transducer application, otherelectronics present in the device do not require high-voltagetransistors, so a simpler cheaper silicon fabrication process with onlylow-voltage transistors can be used in manufacture.

Using low-voltage transistors as the switches does require some extracircuitry to shift the voltage levels of the driving switch waveformsfrom near ground to near the increasing voltage levels of each stage,possibly using more switched capacitors to level-shift these voltages.However such circuitry is relatively simple.

In steady state, the voltage waveforms in each part of the circuit andacross each transistor are predictable, and excessive voltages can beavoided by appropriate design.

However in other scenarios, for example in power-down, it is hard topredict all possible power-down transients, as these will depend on thetiming and the speed of the powering down of the supply, and the outputof any clock waveform generator in all possible cases. If the supplydisappears suddenly, the switches may find themselves stuck in onephase, or possibly all turned off. In such a case, at least some of thecapacitors may be at high voltages, and only decay to ground graduallyvia small leakage currents associated with junctions to which they areconnected. But processing variations across a circuit, or differences incircuit design at different nodes may result in some node voltagesdecaying faster than others. For example the final capacitor is oftenmuch larger than the others to reduce ripple on Vout, so this may decaymore slowly, unless the applied load is still taking current. This isillustrated in FIG. 1 c, which shows the voltage levels at one clockphase (solid line) and the voltage decay a certain time later (dot-dashline). In this example C3 is shown as decaying to ground much quickerthan C4, perhaps also due to a minor defect in a junction coupled to C3increasing its leakage to ground. In this example a large voltagedevelops between C3 and C4, across switch block SS4.

Such a large voltage may damage constituent elements of SS4, forinstance the voltage may be above the breakdown voltage of anytransistor elements and exceeding the breakdown voltage may stress thetransistors to breaking point, especially if repeated often. Such powercycling is becoming more prevalent as more complex power managementschemes are employed in portable equipment to reduce power consumption.Also as silicon technology evolves to use smaller transistorseconomically, there is generally less safety margin possible, so thetransistors are more fragile than in older technologies.

Similar issues of possible high voltages may arise not only across theswitch elements, but also in the capacitor level-shifting circuits LS1to LS4. Internal voltages in such circuits may not be predictable inpower down scenarios and again large voltages may lead to lifespanreduction or failure.

Further if the device does not power down properly, there may bepeculiar transients on power-up which could again cause damage to thedevice.

There is thus a requirement for some means of ensuring that excessivevoltages do not develop across low-voltage elements in these chargepumps, especially at nodes that are capacitively pumped and so may notalways be directly driven high or low, and may be at high voltagesbefore power down.

It is therefore an aim of the present invention to provide a charge pumpcircuit which mitigates at least some of the aforementioneddisadvantages.

SUMMARY OF THE INVENTION

According to a first aspect of the present invention, there is provideda charge pump circuit comprising: at least one component that may bedamaged by voltage transients above the normal operating voltages of thecircuit; and a leakage element electrically connected in parallel withsaid at least one component so as to protect said component from voltagetransients above the normal operating voltages of the circuit.

The present invention provides a leakage element connected in parallelwith a circuit component of the charge pump circuit in order to protectsaid circuit component from voltage transients which are above thenormal operating voltages of the circuit. As described above a chargepump circuit may be designed using transistors that are capable ofoperating at the normal operating voltages of the charge pump, but thatcertain events may lead to voltage transients which generate greaterthan normal applied voltages that may damage the transistors. Thecircuit component may therefore comprise a transistor, and the inventionwill be described in relation to protection of transistors. However theprinciples of the invention apply to protecting other circuit componentsthat may be damaged by such voltage transients.

A leakage element is an element that, whilst generally having theproperty of preventing current flow (at least in one direction)nevertheless allows a small current to leak through the barrierpresented by the leakage element. The leakage elements used in thepresent invention are thus arranged to allow a leakage current to flow,i.e. a current which, in normal operating conditions is not significantenough to prevent satisfactory operation of the charge pump but, in theevent of a developing voltage transient, allows sufficient current toflow to maintain a safe applied voltage. The skilled person willappreciated that leakage elements are high impedance elements, at leastas regards current flow from a higher voltage to a lower voltage stageof the charge pump, so as to prevent excess current flow in normaloperation.

The leakage element is preferably adapted so as to minimise the chanceof the transistor exceeding its breakdown voltage. The leakage elementis therefore conveniently adapted to allow a leakage current to flow ata voltage below the breakdown voltage of the transistor, i.e. it isadapted to prevent a voltage difference developing across the transistorwhich is greater than the breakdown voltage of the transistor. Thetransistors may, as described above, be relatively low voltage elementsand have a breakdown voltage less than the voltage output of the chargepump circuit.

The leakage element may therefore be adapted to allow a leakage currentof more than 100 pA or more than 1 nA or more than 10 nA or more than100 nA at a voltage less than the breakdown voltage of the transistor.The leakage element may be adapted to allow a leakage current of around1 μA to flow. For instance if the safe operating voltage of thetransistor was around 3V for example, the leakage element may be adaptedto allow a leakage current of greater than 1 nA or greater than 10 nA orgreater than 100 nA or around 1 μA to flow at a voltage around 3V. Sucha leakage current would quickly drain charge from the high voltage stageand maintain charge on the low voltage stage so as to minimise thevoltage difference and hence prevent the voltage across the transistorfrom exceeding the breakdown voltage.

Equally however the leakage element should not allow too great a currentto flow in normal operation. Therefore the leakage element may beadapted to allow a leakage current of less than 1 μA or less than 100 nAor less than 10 nA at the normal maximum operating voltage. For examplethe leakage element may be arranged to have a leakage current of around1 nA at the maximum normal operating voltage difference of the charge,i.e. 2Vin in the example given in the introduction.

The leakage element may comprise a polysilicon diode, i.e. a diodeformed in polycrystalline silicon. As will be described in more detailbelow polysilicon diodes have the desired impedance and leakagecharacteristics. In one embodiment therefore the invention relates to acharge pump circuit having at least one circuit component electricallyconnected in parallel with a polysilicon diode.

Conveniently the polysilicon diode is a p-i-n diode. As described inmore detail later the presence of an intrinsic region can overcome grainboundary effects which arise in a polysilicon diode with n and p regionsin direct contact. However other pin diodes may be suitable for use inthe present invention and in another embodiment the invention provides acharge pump circuit having at least one circuit component electricallyconnected in parallel with a pin diode.

The polysilicon diode may comprise a diode having a single pin (or pn)junction or may comprise a multiple junction composite diode. Such amultiple junction composite diode may comprise a continuous structure ofpolysilicon comprising a plurality of regions of first semiconductortype, being n type or p type, and at least one region of a secondsemiconductor type, being n type or p type and the opposite type to thefirst type, the regions of first semiconductor type and secondsemiconductor type being arranged alternately. The continuous structuremay further comprise a plurality of regions of a third semiconductortype, said third semiconductor type being one or more of substantiallyintrinsic, lightly doped p-type or lightly doped n-type, arrangedbetween respective regions of first semiconductor type and regions ofsecond semiconductor type. There may be a plurality of regions of secondsemiconductor type.

In other words the composite diode is has an alternating arrangement ofn type and p type regions, preferably separated by substantiallyintrinsic regions (formed from intrinsic material or lightly dopedmaterial). Such a composite diode thus comprises a plurality of pn andnp (or pin/nip) junctions. Each such junction can be thought of aseffectively comprising a diode arranged in electrical series with theother junction diodes. By appropriate choice of the number of junctionsthe characteristics of the overall composite diode can be varied tomatch the desired characteristics.

It should be noted that it is possible to use a composite junction diodewhich is symmetric. For instance a composite diode with p and n typeregions arranged alternately and having a p type region at each end (oran n type region at each end) will have an equal number of pn junctionsas np junctions. The dc characteristics of such a device are symmetricand thus the device does not have distinct forward and reverse biasmodes. The term diode as used herein is not therefore restricted todevices which have distinct forward and reverse characteristics

In order to provide the desired characteristic the leakage element maycomprise a plurality of diodes electrically connected across the circuitcomponent to be protected, for example a transistor. For instance theplurality of diodes may be electrically connected in series.

As described above charge pumps typically have a plurality of stages anda transistor may be electrically connected between adjacent stages ofthe charge pump. Conveniently the leakage element is arranged to bereverse biased when the later stage of the charge pump, i.e. the stagecloser to the output, is at a higher voltage than the earlier stage ofthe charge pump. As described above adjacent stages of the charge pumpmay be at the same voltage during one clock period and then at a voltagedifference of say 2Vin during a different clock period, with the voltageon the later stage being higher than that at the earlier stage. Theleakage element is arranged such that it is reverse biased when thelater stage is at a higher voltage than the earlier stage.

Note that, as described above, the leakage element may comprise apolysilicon diode will a symmetric dc characteristic and such a diodedoes not have distinct forward and reverse directions. As used hereinthe term reverse bias means that the diode is arranged so that thecurrent flow under normal operating conditions is the same or less thanthe current flow that would be observed were the diode to be connectedthe other way around.

The transistor protected by the leakage element may be part of aswitching stage for switchably connecting a capacitor of one stage ofthe charge pump to a previous stage of the charge pump or to the inputvoltage to the charge pump. Additionally or alternatively a transistorprotected by a leakage element may be part of a level shifting cell ofthe charge pump circuit. The circuit may have a plurality oftransistors, each electrically connected in parallel with a leakageelement.

The output of charge pump may be electrically connected to a biasingcircuit for biasing a device. The device may be a transducer such as aMEMS microphone. Such a MEMS microphone may be used in a variety ofapplications including, but not limited to an ultrasound imager, a sonartransmitter and/or receiver, a mobile phone or other communicationdevice, a personal desktop assistant, an MP3 player or other personalaudio device or a laptop computer.

In another aspect of the present invention there is provided a method ofprotecting a circuit component in a charge pump from voltage levelshigher than normal operating voltages comprising the step of arranging aleakage element in parallel with said circuit component. The circuitcomponent may comprise a transistor and the transistor may have abreakdown voltage less than the voltage output of the charge pumpcircuit. The leakage element may be adapted to prevent a voltage acrossthe transistor which is greater than the breakdown voltage of thetransistor

The leakage element may comprise a polysilicon diode. The polysilicondiode may be a multiple junction composite diode, comprising acontinuous structure of semiconducting material comprising a pluralityof regions of first semiconductor type, being n type or p type, and atleast one region of a second semiconductor type, being n type or p typeand the opposite type to the first type, the regions of firstsemiconductor type and second semiconductor type being arrangedalternately

The leakage element may comprise a pin diode.

The leakage element may comprise a plurality of diodes connected inseries and/or parallel.

BRIEF DESCRIPTION OF THE DRAWINGS

For a better understanding of the present invention, and to show moreclearly how it may be carried into effect, reference will now be made,by way of example, to the following drawings, in which:

FIG. 1 a illustrates an example charge pump circuit;

FIG. 1 b illustrates the voltage levels experienced at various nodes ofthe circuit of FIG. 1 a;

FIG. 1 c shows an example of voltage levels that may be developed onpower down in the circuit of FIG. 1 a;

FIG. 2 a illustrates a charge pump circuit according to an embodiment ofthe present invention;

FIG. 2 b illustrates an example of voltage levels that may be developedon power down in the embodiment shown in FIG. 2 a;

FIG. 2 c illustrates the leakage characteristics of a diode suitable foruse in the present invention;

FIG. 3 a illustrates a poly diode suitable for use in the presentinvention;

FIG. 3 b illustrates the current voltage characteristics of the polydiode shown in FIG. 3 a;

FIG. 3 c shows a composite poly diode suitable for use in the presentinvention;

FIG. 4 illustrates a single stage of a charge pump showing the circuitassociated with the switching and level shifting stages;

FIG. 5 illustrates an embodiment of the invention for protecting thelevel shifting stage; and

FIG. 6 illustrates the charge pump circuit of the present inventionarranged to provide a biasing voltage for a MEMS transducer.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

FIG. 2 shows a charge pump architecture embodiment according to anaspect of the invention. The circuit operates in the same manner, and issimilar in arrangement to a convention charge pump circuit such as thatshown in FIG. 1, but with the addition of leakage elements, diode clampdevices D1 to D4, across consecutive switch blocks SS1 to SS4, betweenconsecutive pump capacitors C1 to C4. The diodes clamp devices eachcomprise a diode arranged to be reverse biased in terms of current flowfrom a higher voltage stage of the charge pump to a lower voltage stageof the charge pump.

As described above, in normal operation, the voltage difference acrosseach switching stage will be either zero or 2Vin. Thus, except for brieftransients, these diodes will either be at zero bias or reverse biasedby 2.Vin. In the event of a power down the capacitors will begin todischarge. In the example described above in relation to FIG. 1 c aleakage current associated with capacitor C3 caused it to dischargefaster than capacitor C4—leading to a high voltage difference. In thepresent invention however, as shown in FIG. 2 b, provided that the diodeleakage current characteristics are such that it presents more leakagethan is likely in even a mildly defective junction on a relevant diodenode, then the clamp diode leakage will dominate and prevent a largevoltage building up between C3 and C4.

In other words the diodes D1-D4 act to protect each switching stageduring a power-down shut-off mode of operation or other cause of voltagetransients. When the reverse bias voltage across each switching stagerises, the respective diode D reacts accordingly so as to divert currentthrough the diode, thus allowing excess charge on the pump capacitor Cto be discharged and limiting the voltage across each switching stage.

It will be appreciated by those skilled in the art that the diodes D1-D4will exhibit some reverse leakage current during normal operation, wherethey are reverse biased by 2Vin for about half the operating time.Therefore, the reverse leakage current characteristics of such diodes Dare preferably chosen such that: during the normal mode operation theoutput bias voltage Vb of the charge pump is not significantly reduced;and during the power down/off mode of operation the stages CP1-CPN aredischarged in a sufficient and predictable manner.

FIG. 2 c illustrates these constraints. When reverse biased by 2Vin, thediode must pass less than a current, Idmax, that is the maximum currentthat is tolerable in terms of lack of efficiency or potentially limitingthe output voltage by approaching the current delivering capability ofthe capacitors at the normal clock frequency. On the other hand, thediode must pass the maximum expected extraneous pull-down leakagecurrent Ileak before reaching a voltage Vsafe corresponding to themaximum permissible voltage stress across the switching stage beforereliability is compromised.

A suitable diode, which has the above mentioned characteristics, is apolysilicon diode. FIG. 3 a shows a known structure of such apolysilicon diode element 20 (referred to hereinafter as a “polydiode”).

The poly diode element 20 is disposed on a silicon dioxide layer 22,previously disposed on a silicon substrate 24. The poly diode element 20itself comprises an n-type region 26 forming a p-n junction with ap-type region 28. If the n-type and p-type polysilicon regions aretouching, low reverse breakdown voltages and/or high leakage of currentare observed, as the n- and p-type regions 26, 28 are polycrystalline,creating effects at the grain boundaries. The low reverse breakdownvoltages and/or high leakage of current can be adjusted by interposingan intervening drift region 30 of substantially intrinsic semiconductormaterial.

The n-type region 26 is electrically connected to an electrode 32, andthe p-type region 28 is electrically connected to another electrode 34through contact holes etched in an overlying insulating dielectric layer35.

Typically this structure will be manufactured by first depositing alayer of intrinsic material, etching away superfluous material to leavea polysilicon region for the whole diode, than selectively implanting ordiffusing n or p dopant on the respective portions of this intrinsicmaterial. The insulating layer 35 is then deposited, and holes etchedinto it to accommodate the vertical elements of the metal electrodes 32,34 which are then deposited to fill the holes and in patterns on thesurface to connect with other circuit elements (not illustrated).

FIG. 3 b shows the current-voltage curves for a typical poly diode suchas shown in FIG. 3 a. In forward bias, the current asymptotes toIs.exp(Vd/(2.kT/q), so Vd=(2kT/q)In(|Id|). In reverse bias, the currentasymptotes to Is.exp(Vd/(8.kT/q) so Vd=(8kT/q)In(|Id|). At zero biasthese current components cancel to give zero net current.

For a convention silicon pn diode, where the currentId=Is.exp(Vd/(kT/q), the current Id increases by a factor of 10 forevery 60 mV increase in Vd. For a reverse biased poly diode such asshown in FIG. 3 a, since Id=Is.exp(Vd/(8.kT/q), the extra factor of 8 inthe exponent leads to the poly diode requiring a near 500 mV increase inapplied voltage Vd to increase current by a factor of 10. The saturationcurrent Is of a typical p-i-n (or n-i-p) poly diode is of the order of 1pA, so it only leaks 10 pA by Vd˜500 mV and 1 nA by Vd˜1.5V, and 1 μA byVd=3.0V. Thus the use of a reverse biased poly diode DR enables acontrolled reverse leakage current to be obtained.

A poly diode, having the characteristics shown in FIG. 3 b, would besuitable for an application where Vin=0.75V, using 0.18 μm transistorswith a reliability limit Vsafe of 3V across them. The operating leakagecurrent would be 1 nA at 1.5V Vd, yet the diode could clamp currents ofup to 1 μA to 3V.

The diode clamps may comprise a single poly diode, or a plurality ofpoly diodes electrically connected to achieve a desired operatingcharacteristic. When using a plurality of poly diodes to obtain aparticular reverse leakage current characteristic, the poly diodes maycomprise a series of individual poly diodes connected in series.

For example if each diode clamp consisted of two poly diodes in series,each with the I-V characteristic of FIG. 3 c, then the overall clampwould pass the stated currents with double the respective appliedvoltages. Thus the clamp would pass 1 nA with 3V applied, 1 μA with 6Vapplied, rendering it suitable for use in a low-current charge pump withVin=1.5V, using 0.35 μm transistors with a reliability limit Vsafe of 6Vacross them. The operating leakage current would be 1 nA at 3V Vd, yetthe diode could clamp currents of up to 1 μA while ensuring less than 6Vwas applied across the protected switch transistor.

The diode may also comprise a composite poly diode, having a continuousstrip of polysilicon with a plurality of p-n (or n-p) junctions arrangedto form a series of poly diode elements on a single substrate. Such acomposite diode structure is described in more detail in our co-pendingapplication, applicant's reference P1200 GB00 (P111707 GB00). FIG. 3 cshows a composite diode 38 which could be used where similar elementshave the same numerals as used in FIG. 5 a. A continuous strip ofpolysilicon 36 comprises a plurality of doped regions having alternatingn-type regions 26 and p-type regions 28. As mentioned above, directjunctions between n- and p-type regions can cause leakage, and thereforein the illustrated embodiment the n- and p-type regions are preferablyseparated by regions 30 of substantially intrinsic semiconductormaterial (for example, polysilicon). It is noted, however, that theregions 30 may have some degree of light doping.

Use of a composite poly diode as a leakage element in a diode clampaccording to the present invention, either alone or connected to otherdiodes (whether single junction diodes such as shown in FIG. 3 a orother composite diodes) allows the characteristics of the diode clamp tobe tailored for the particular application. As mentioned above polydiodes are particularly suited to the present application due to theirleakage characteristics.

It will be noted however that the invention is equally applicable to anysemiconductor material for realising the diode, whether composite orotherwise, for example re-crystallised silicon or other semiconductormaterial(s), having a high impedance and capable of conducting therequired reverse bias current.

As mentioned above any voltage transients developed during power downmay cause damage not only to the switching stages, SS1 to SS4 in FIG. 1a but also to the level shifting stages LS1 to LS4. FIG. 4 shows anexample circuit associated with a single charge pump stage including aswitching stage SS 401 and a level shifting stage LS 402 correspondingto those shown in FIG. 1 a. The switching stage SS 401 comprises a passPMOS transistor TS. The level shifting stage comprises PMOS transistorsTa, Tb and capacitors Ta and Tb and an inverter.

The switch transistor TS in the switching stage 401 may be protectedfrom high voltages developed in power down situations and the like byconnecting a diode in parallel with the transistor TS as describedabove. However there is also a desire to protect the circuitry of thelevel shifting cell 402.

The Level shifting cell 402 uses two non-overlapping, anti-phase clocks(CK and inverted CK). Transistors Ta and Tb are successively switched onand off in order to charge capacitors Ca and Cb to the input voltage.During certain modes of operation, for example a power-down mode ofoperation, the different rates of discharge in the various stages of thecharge pump circuit can lead to the reverse breakdown voltage oftransistors Ta and Tb being exceeded, thereby damaging the transistorsTa and TB.

FIG. 5 illustrates the circuit associated with a charge pump stage ofFIG. 4 but with a means of protecting the transistors in the levelshifting cell according to a another embodiment of the presentinvention. Respective diode clamps Da & Db are connected across switchtransistors Ta & Tb. Each diode clamp comprises at least one diode,which may be a poly diode. The cathodes of diodes Da and Db areconnected to the output voltage node and their anodes are connected tothe high sides of the level shift pump capacitors Ca and Cb. In otherwords each diode is connected in parallel with its respective transistorand reverse biased with respect to current flow from Vout of the chargepump stage.

Again it is preferable that the diodes, Da and Db are leaky diodes. Thatis to say the diodes, Da and Db exhibit a relatively small reverseleakage current characteristics such that if a high voltage starts todevelop at Vout, the respective diodes, Da and Db will discharge therelevant pump capacitor connected to Vout such that voltage across thetransistors Ta and Tb does not exceed the breakdown voltage of thetransistors. The diode clamps Da and Db therefore conveniently compriseone or more poly diodes.

The charge pump described above may be used in a variety of applicationsand in particular is suitable for generating a voltage required to biasa transducer such as a MEMS transducer like a microphone, as may be usedin a portable device.

FIG. 6 shows how the charge pump circuit of the present invention couldbe implemented into a device and illustrates a schematic diagram of aMEMS device 99 comprising a MEMS transducer 100 and an electroniccircuit 102. The MEMS transducer 100 is shown as being formed on aseparate integrated circuit to the electronic circuit 102, the two beingelectrically connected using, for example, bond wires 112, 124. The MEMStransducer 100 comprises a MEMS capacitor C_(MEMS) having first 118 andsecond 120 plates that are respectively connected to first 114 andsecond 122 bond pads.

The electronic circuit 102 comprises a charge pump circuit 104 accordingto the present invention, such as the one shown in FIG. 2 a. The devicecircuitry also comprises a resistor 106 and a reservoir capacitor 108,an amplifier 128, a bias circuit 131, third 110 fourth 126 and fifth 130bond pads and an optional digital-to-analogue converter (DAC) 132 withassociated sixth bond pad 134.

However the charge pump may be used for any application requiring avoltage level to be supplied and for example could be used in biasingthe gate of a MOS transducer. Thus the charging circuit could be used ina number of different devices including, but not limited to anultrasound imager, a sonar transmitter and/or receiver, a mobile phoneor other communication device, a personal desktop assistant, an MP3player or other personal audio device or a laptop computer.

It should be noted that the above-mentioned embodiments illustraterather than limit the invention, and that those skilled in the art willbe able to design many alternative embodiments without departing fromthe scope of the appended claims. The word “comprising” does not excludethe presence of elements or steps other than those listed in a claim,“a” or “an” does not exclude a plurality, and a single processor orother unit may fulfil the functions of several units recited in theclaims. Any reference signs in the claims shall not be construed so asto limit their scope.

What is claimed is:
 1. A charge pump circuit having a plurality ofstages comprising: at least one circuit component that comprises atransistor and that may be damaged by voltage transients above thenormal operating voltages of the circuit, wherein said circuit componentis part of a switching stage for switchably connecting a capacitor ofone stage of the charge pump to a previous stage of the charge pump; anda leakage element electrically connected in parallel with said at leastone circuit component so as to protect said circuit component fromvoltage transients above the normal operating voltages of the circuit,wherein said leakage element is configured to prevent a voltagedifference developing across the transistor which is greater than thebreakdown voltage of the transistor; wherein said leakage element isarranged to be reverse biased when said one stage of the charge pump isat a higher voltage than said previous stage of the charge pump.
 2. Acircuit as claimed in claim 1 wherein said leakage element is adapted toallow a leakage current to flow at a voltage below the breakdown voltageof the transistor.
 3. A circuit as claimed in claim 1 wherein saidtransistor has a breakdown voltage less than the voltage output of thecharge pump circuit.
 4. A circuit as claimed in claim 1 wherein theleakage element is adapted to allow a leakage current of more than 100pA or more than 1 nA or more than 10 nA or more than 100 nA at a voltageless than the breakdown voltage of the transistor.
 5. A circuit asclaimed in claim 1 wherein the leakage element is adapted to allow aleakage current of more than 100 pA or more than 1 nA or more than 10 nAor more than 100 nA at a voltage which is substantially the breakdownvoltage of the transistor.
 6. A circuit as claimed in claim 1 whereinthe leakage element is adapted to allow a leakage current of less than 1μA or less than 100 nA or less than 10 nA at the normal maximumoperating voltage.
 7. A circuit as claimed in claim 1 wherein saidleakage element comprises a polysilicon diode.
 8. A circuit as claimedin claim 7 wherein said leakage element comprises a p-i-n diode.
 9. Acircuit as claimed in claim 7 wherein said polysilicon diode comprises amultiple junction composite diode.
 10. A circuit as claimed in claim 9wherein said multiple junction composite diode comprises a continuousstructure of polysilicon comprising a plurality of regions of firstsemiconductor type, being n type or p type, and at least one region of asecond semiconductor type, being n type or p type and the opposite typeto the first type, the regions of first semiconductor type and secondsemiconductor type being arranged alternately.
 11. A circuit as claimedin claim 10, wherein said continuous structure further comprises aplurality of regions of a third semiconductor type, said thirdsemiconductor type being one or more of substantially intrinsic, lightlydoped p-type or lightly doped n-type, arranged between respectiveregions of first semiconductor type and regions of second semiconductortype.
 12. A circuit as claimed in claim 9 wherein said multiple junctioncomposite diode comprises a plurality of regions of second semiconductortype.
 13. A circuit as claimed in claim 1 wherein said leakage elementcomprises a plurality of diodes electrically connected across saidcircuit component.
 14. A circuit as claimed in claim 13 wherein saidplurality of diodes are electrically connected in series.
 15. Anapparatus having a charging circuit as claimed in claim 1 wherein theapparatus is one of an ultrasound imager, a sonar transmitter and/orreceiver, a mobile phone or other communication device, a personaldesktop assistant, an MP3 player or other personal audio device or alaptop computer.
 16. A circuit as claimed in claim 1 wherein at leastone said circuit component is a transistor which is part of a levelshifting cell.
 17. A circuit as claimed in claim 1 having a plurality oftransistors, wherein each of said plurality of transistors iselectrically connected in parallel with a leakage element.
 18. A circuitas claimed in claim 1 wherein the output of charge pump is electricallyconnected to a biasing circuit for biasing a device.
 19. A circuit asclaimed in claim 18 wherein said device is a transducer.
 20. A circuitas claimed in claim 19 wherein said transducer is a MEMS microphone. 21.A method of protecting a circuit component in a charge pump having aplurality of stages from voltage levels higher than normal operatingvoltages comprising the step of arranging a leakage element in parallelwith said circuit component, wherein the circuit component comprises atransistor, which is part of a switching stage for switchably connectinga capacitor of one stage of the charge pump to a previous stage of thecharge pump, and the leakage element is configured to prevent a voltagedifference developing across the transistor which is greater than thebreakdown voltage of the transistor, wherein said leakage element isarranged to be reverse biased when said one stage of the charge pump isat a higher voltage than said previous stage of the charge pump.
 22. Amethod as claimed in claim 21 wherein said transistor has a breakdownvoltage less than the voltage output of the charge pump circuit.
 23. Amethod as claimed in claim 21 wherein said leakage element comprises apolysilicon diode.
 24. A method as claimed in claim 23 wherein saidleakage element comprises a pin diode.
 25. A method as claimed in claim23 wherein said polysilicon diode comprises a multiple junctioncomposite diode.
 26. A method as claimed in claim 25 wherein saidmultiple junction composite diode comprises a continuous structure ofsemiconducting material comprising a plurality of regions of firstsemiconductor type, being n type or p type, and at least one region of asecond semiconductor type, being n type or p type and the opposite typeto the first type, the regions of first semiconductor type and secondsemiconductor type being arranged alternately.
 27. A method as claimedin claim 21 wherein said leakage element comprises a plurality of diodesconnected in series and/or parallel.